1. Field of the Invention
An aspect of the present invention relates to a method of determining capability and quality of foundation pipes and a method of designing foundation piles and an apparatus which is applied to the methods for measuring characteristics of the ground. Another aspect of the present invention relates to a method of drilling a hole of a shape designed for foundation piles such as cast-in-situ piles to support a structure and an apparatus for the method.
2. BACKGROUND ART
There are earth dril methods, overall casing methods, reverse circulation drill methods, etc. as cast-in-situ pile methods. In each method, a drilling machine drills a hole in a predetermined groud of a predetermined diameter by a predetermined depth. After the drilling machine is pulled out of the ground, a suspended termie is put in the borehole to remove slime at the bottom of the borehole. Then, a suspended rebar cage is moved down to the bottom of the borehole, and ready-mixed concrete is injected into the hole to fill the hole, while the tremie is being lifted up. Hardening of the concrete results in a cast-in-situ pile. Meanwhile, the foundation pile may be made with a prefabricated pile by filling the borehole with bottom consolidation cement slurry and inserting the prefabricated pile such as a concrete pile, instead of using the rebar cage. However, there have been some problems described below.
The supporting capability of the foundation piles, which may be the cast-in-situ pile or the prefabricated pile, for a structure is ordinarily determined in the following way.
As size, shape, etc of the structure on the predetermined site are designed, the vertical load, the lateral force by an earthquake or a wind, and the bending moment applied to the foundation pile are accordingly determined. A geological survey in the predetermined site is performed, foundation piles capble of enduring the above-mentioned forces are sought, and the kind of the foundation piles (the cast-in-situ pile or the prefabricated pile), the diameter of the pile, the length (depth) of the pile, the way of construction and the design bearing capacity are determined. According to the kind of structures constructed, the allowable settlement and the allowable lateral displacement, namely, the design deformation, after construction of the structure are also taken into consideration to determine the foundation pile and the way of construction.
However, the bearing capacity and deformation of the foundation piles considerably depend on the soil condition of the ground in which the foundation pile is to be placed, and they are not known until the foundation pile is placed in the predetermined ground and load is practically applied to the pile (i.e., a loading test). It takes many days to carry out the loading test, considering the entire term necessary for constructing the structure, and it is impossible to perform the loading test on every one of the piles, considering the term and the costs necessary for the construction. A cast-in-situ pile has in general a large bearing capacity, so that the loading test costs for a cast-in-situ pile become prohibitive.
Accordingly, the foundation pile is designed by an indirect method where its bearing capacity and deformation are determined from empirical formulae which have been obtained by analyzing data of existing loading tests based upon geological survey data such as SPT-N values in the ground at the site.
However, with regard to application of the aforementioned indirect method, there is the disadvantage that when the cast-in-situ pile is made, namely, a hole for the pile is drilled by a drilling machine such as an earth drill, the wall of the borehole may crumble due to the vertical movement of the drilling machine, or the bearing capacity of the ground is reduced due to the decompaction and disturbance of the bottom of the borehole, so that the cast-in-situ pile can not be made as expected and specified in design.
The geological survey itself is restricted by time and cost and carried out only for a few parts of the vast site, where its soil condition may be heterogeneous, to be provided with lots of foundation piles. The bearing capacity of each of the many unsurveyed foundation piles is found by applying the above-mentioned soil condition data to the entire site, so that obtained values for the bearing capacity are inaccurate, and applying those values to practical construction is dangerous.
The empirical formula itself has the disadvantage explained hereinafter. In general, the loading test is performed in the condition that the foundation pile provided in the actual ground is loaded on its top with a yield load Py (the pile or the ground varies from an elasto-plastic state to a plastic state) or with an ultimate load Pu (the pile or the ground fails), as shown in FIG. 26. For the design bearing capacity, the deformation of the foundation pile is taken into consideration, and a smaller value (1/2) Py or (1/3) Pu, is employed for practical provision of the foundation pile. In other words, the construction is uneconomically performed, taking an excessive safety factor.
The empirical formula is obtained by analyzing several loading tests as stated above. FIG. 27 shows a graph in which the axis of abscissa represents the bearing capacity data of the pile obtained by the practical loading test and the axis of ordinate represents the bearing capacity of the pile calculated with empirical formulae based upon the geological survey data at the respective grounds sites of the loading tests. Data for a number of sites are plotted in the graph.